The aims of this study are: 1) To determine the prevalence of decreased BMD in children with active or inactive graft vs. host disease; 2) To correlate the BMD with duration of steroid and cyclosporine use (which is correlated with duration of GVHD), adjusting for other determinants of BMD, such as obesity, dietary intake of calcium and vitamin D, pubertal stage and physical activity; 3) To measure and compare the changes in bone mineralization over time in children with decreased bone density randomized to treatment with either calcium/vitamin D, or placebo.